1. Field of the Invention
The present invention relates to a chain adjustment apparatus for a motorcycle. More particularly, the present invention relates to a chain adjustment apparatus which enhances a coupling rigidity of an adjustment member, through which a rear wheel axle is passed, with an end piece member attached to an end portion of a rear fork of a motorcycle.
2. Description of the Background Art
There is a known support structure for a rear wheel axle of a motorcycle. An example of such support structure is disclosed in the Japanese Patent Document JP-A No. 563-48691. FIG. 17 is a perspective view showing a support structure for a rear wheel axle according to the related art disclosed in the Japanese Patent Document JP-A No. S63-48691.
As shown in FIG. 17, an end piece member 108 is attached to the rear end portion of a rear fork 101 for supporting a rear wheel axle 103. The end piece member 108 is formed in a C-shaped cross section, and includes an upper face wall 108a, a side face wall 108b, and a lower face wall 108c. The side face wall 108b has an elongated opening 109 formed therein. An adjustment member 110 is fitted into a recess formed by a portion of the C-shaped end piece member 108 so as to freely move in the longitudinal direction, and positioning of its front end is effected by an adjustment bolt 111.
The rear wheel axle 103 having a long bolt-like shape extends through the adjustment member 110 and the elongated opening 109. Fastening a shaft end portion of the rear wheel axle 103 with a nut (not shown) causes the adjustment member 110 to be tightly fixed onto the side face wall 108b of the end piece member 108, and loosening the nut causes the adjustment member 110 to move in the longitudinal direction together with the rear wheel axle 103 to thereby adjust the tension of the chain. It is desired that the rear wheel axle 103 is supported firmly in an appropriate position during driving operation of a motorcycle. Accordingly, it is expected that the adjustment member 110 is firmly fastened the end piece member 108 for providing sufficient rigidity of fastening.
However, according to the art disclosed in the Japanese Patent Document JP-A No. S63-48691, while the adjustment member 110 is brought into pressure contact with the side face wall 108b of the end piece member 108 when fastening the rear wheel axle 103 for fixation, the force with which the adjustment member 110 is fastened to the end piece member 108 is provided by the axial force applied at the time of fastening of the rear wheel axle 103, and this axial force is applied to the adjustment member 110 from a head portion 103a and a flange 103b, thereby effecting friction coupling due to pressure contact solely by the surface contact portion between the adjustment member 110 and the side face wall 108b. 
Moreover, considering the ease of movement at the time of adjustment, the contact between the upper and lower portions of the adjustment member 110 with the upper face wall 108a and lower face wall 108c of the end piece member cannot contribute to the coupling between the upper and lower portions of the adjustment member 110 with the upper face wall 108a and lower face wall 108c of the end piece member. In addition, a slight gap exists between a shaft portion 103c of the rear wheel axle 103 and a shaft hole in the adjustment member 110. Such slight gap between the shaft portion 103c of the rear wheel axle 103 and the shaft hole in the adjustment member 110 may be disadvantageous for achieving a desired coupled rigidity or for enhancing coupling rigidity.
Therefore, the axial force on the rear wheel axle 103 is applied to the adjustment member 110 from only the head portion 103a, and the rigidity of fastening of the adjustment member 110 to the end piece member 108 is enhanced by increasing a thickness of the adjustment member 110 and the end piece member 108, or the like. The rear wheel axle 103 includes a flange 103b a hexagonal portion 103e (FIG. 17).
It is also desired to enhance the coupling rigidity in order to prevent deformation of the support portion of the rear wheel axle 103. As a result, the device is enlarged in size, often leading to an increase in weight and cost.
The present invention has been made to overcome such drawbacks of the existing chain adjustment apparatus. Accordingly, it is one of the objects of the present invention to provide a chain adjustment device for firmly coupling the adjustment member and the end piece member together at the support portion of the rear wheel axle to thereby enhance coupling rigidity.